The present invention relates to switching mode power supplies (SMPS), and more particularly to the switching mode power supplies with low cost, low standby power, and versatile protection features.
Regulated power supplies are indispensable in modern electronics. For example, desktop and laptop computers often need to receive unregulated power input from various outlets and provide regulated power supplies on the motherboard to the CPU, memories, and periphery circuitry. Regulated power supplies are also used in a wide variety of consumer applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
In general, a power supply can be regulated using a linear regulator or a switching mode controller. A linear regulator maintains the desired output voltage by dissipating excess power. In contrast, a switching mode controller rapidly switches a power transistor on and off with a variable duty cycle or variable frequency and provides an average output that provide the desired output voltage. Switching mode power supplies (SMPS) have the advantages of smaller size, higher efficiency and larger output power capability, and are widely utilized in mobile phone chargers, notebook computer adapters, and other fields. In recent years, green power supplies are emphasized, which require higher conversion efficiency and especially lower standby power consumption.
Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are two control architectures of switching mode power supplies. In PWM control circuits, the duty cycle of the control pulse is used to control the output of the power supply. In a PFM controlled switching mode power supply, the switching frequency can be controlled in response to load conditions.
FIG. 1 illustrates a conventional switching mode power supply (SMPS) 100 which includes a full-wave bridge rectifier 21, a transformer 30 with a primary winding 31, a secondary winding 38, and an auxiliary winding 32. SMPS 100 also has a controller 23, filter capacitors 22, 24, and 35, and rectifying diodes 25 and 34. When the system is in normal operation mode, the operating power of controller 23 is provided by auxiliary winding 32, diode 25, and capacitor 24. But at system startup, no energy will be delivered from auxiliary winding 32 because controller 23 will not provide switching signals to power transistor 28. So in the startup phase, capacitor 24 can only be charged by the startup circuits of controller 23.
The startup circuits in controller 23 includes high voltage startup transistors 40 and 41, a switch 42, a startup current limit resistor 43, and startup current limit transistors 44, 45, and 46. Controller 23 also has an under voltage lock out circuit including a comparator 48, a switch 47, and voltage divider resistors 49 and 50, as well as switching mode power supply control unit 51 (PWM CTRL). When Vcc rises above a threshold voltage, comparator 48 operates to maintain switch 42 off, which will cut off the startup current during system normal operation. Control unit 51 regulates output voltage Vout in response to a voltage feedback signal from the FB pin through voltage divider resistors 26 and 27 and a current sense signal from the CS pin through the voltage drop across resistor 29.
Depending on the application, on-chip high voltage start up transistors 40 and 41 may require junction breakdown voltages higher than 500V. The on-chip implementation of high voltage startup transistors 40 and 41 can be expensive because the integration of high voltage active devices with the low voltage control circuits requires expensive substrate or epitaxial semiconductor materials and several extra photolithography steps. Moreover, the high voltage (HV) pin associated with the high voltage transistors can be vulnerable to electrical static discharge (ESD) due to the limited size of the on-chip high voltage transistors.
From the above, it is seen that even though conventional switching mode power supplies (SMPS) are widely used, they suffer from many limitations. Therefore, improved switching mode control circuits and power supplies are desired.